In the art of drilling boreholes in the earth, difficulties are encountered when using jet drilling techniques to advance a borehole, particularly when drilling radial boreholes from a vertical borehole. In jet drilling, a high pressure jet of fluid, often a slurry of water and abrasive material, is directed at a formation to be penetrated from a nozzle at the end of a drill string. The nozzle is moved slowly through the formation with the jet of abrasive slurry cutting away the rock ahead of the nozzle. Abrasive slurry is returned to the surface. Examples of jet drilling techniques used in the drilling of radial boreholes are described in U.S. Pat. No. 5,373,906 of Braddick and U.S. Pat. No. 4,763,734 of Dickinson et al. In these patents jet drilling systems are described in which a jet nozzle at the end of a length of coiled tubing is advanced around a short radius whipstock to move from a vertical to horizontal orientation and thus be able to drill multiple radial boreholes from a central borehole.
One difficulty encountered in jet drilling through a short radius whipstock is that the coiled tubing tends to jam in the whipstock if it is advanced too slowly, and to overtake its ability to cut into the formation if advanced too quickly. In situations where the range of acceptable rate of advance is small, for example when drilling in geological targets with high unconfined compressive strength, prior devices for advancing the coiled tubing, for example as described in U.S. Pat. No. 4,763,734 of Dickinson et al, have been tested and found unsatisfactory.
In addition, previously known hydraulic actuators used for the control of advance of jet drilling apparatus, such as the Precise Pipe Control Unit of Halliburton Services Limited of Calgary, Canada, are not believed to have the degree of control required to advance a jet nozzle at a satisfactorily controlled rate and therefore have not been used for this purpose.
A further lacking of previously known drilling jet drilling systems is their inability to remove casing and subsequently underream the formation to make an enlarged borehole suitable for placement of a whipstock. The inventor has found that this difficulty arises from the lack of ability to suitably control the rate of advance and rate of rotation of the jet nozzle. For example, so far as known to the inventor, the Precise Pipe Control Unit of Halliburton has only been used for cutting slots in casing and not for section milling and underreaming. The consequence of failure to precisely control section milling or underreaming is that if any casing is left behind, or the formation is not underreamed fairly uniformly, the subsequent drilling operations cannot be carried out. For example, underreaming cannot be carried out if section milling is incomplete and a whipstock cannot be installed if the underreaming is incomplete. Tripping out of a borehole is expensive, so it is desirable to complete each task sequentially.